The Passacaglia for Violin and Viola is a musical piece composed in the late 19th century by Johan Halvorsen. based on an earlier work, the last movement of the Harpsichord Suite in G Minor HWV 432, by Georg Friedrich Händel. It is a popular duet that takes the [1] from of the Passacaglia, a set of variations over a common baseline, and refracts it through the prism of Romantic virtuoso string writing.
Data, together with his "mother" Juliana Tainer, performed the Passacaglia for Violin and Viola during a concert aboard the USS Enterprise-D in 2370. Data played the violin, Tainer the viola. (TNG: "Inheritance") It plays a central role in the episode story because Data notices that his mother exactly replicated her Intonation from rehearsal to performance, a feat impossible for even the most masterful human players, lending him to correctly believe that she is an android even superior to himself.